1. Field of the Invention
The present invention relates to a bearing with a preload arrangement, and a method of preloading a bearing, especially a variable pitch propeller bearing.
2. Description of the Related Art
The increase in the power handling characteristics and diameters of propeller systems makes it increasingly necessary to preload the bearings within the propeller hub which allow the propeller blades to change pitch. The preload serves many purposes, the primary one of which is to ensure that all of the bearing elements are kept stable during all flight conditions. Preloading also serves to align the blade root within the propeller hub and, in applications having removable blades, also provides a means for locking the blades into the hub.
A known propeller blade root assembly typically comprises a split upper inner race which faces radially outward of the propeller blade and is inclined with respect to the axis of the blade so as to face slightly towards the propeller root. An outer bearing race is disposed generally radially outwards of the inner bearing race and has an inwardly facing bearing surface which is slightly inclined so as to be generally parallel to the bearing surface of the inner bearing race. Motion of the outer bearing race along the axis of the propeller varies the distance between the bearing surfaces of the inner and outer bearing races and this feature is exploited to preload the bearings. The outer bearing race has an outwardly facing screw thread located thereon which cooperates with a preload nut which is wound down to bear against the hub, either directly or indirectly (to prevent fretting). The winding down of the nut causes the outer bearing race to be moved axially of the propeller blade and to preload the propeller blade bearings. The preload is applied via a spanner acting on the nut and considerable effort may be required. The friction between the nut, the thread, and the hub makes it difficult to determine the torque required to obtain a desired preload. This often necessitates the performing of tests in order to determine what preload had been achieved.